Dental Restoration Preform and Method of Making the Same

ABSTRACT

A machinable preform for shaping into dental restorations is described that comprises material having suitable strength for use in dental applications without requiring further processing after shaping to strengthen the material (such as sintering). In one embodiment, a preform is comprised of a machinable dental material having a Vickers hardness value in the range of 4HV GPa to 20HV GPa, and comprises a body and a stem that extends from the outer surface of the body that supports the body during shaping. A method for making the machinable preform, and a kit comprising a machinable preform and a grinding tool, are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/200,532, filed on Aug. 3, 2015, the entiretyof which application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Ceramic materials known for use in the field of dentistry provide highstrength restorations such as crowns, bridges, and the like. Someceramic materials have flexural strength values exceeding 800 MPa whenfully sintered, resulting in restorations that are resistant tochipping, breakage and wear. Material advances provide enhancedaesthetics in color and translucency while maintaining acceptablestrength, and restorations may be manufactured from these materials in acost effective manner.

Dental restorations created by computer assisted design processes may bemilled by CAM processes from porous ceramic materials in the green orpre-sintered ceramic stage, using an enlargement factor to accommodatereduction in overall size upon heating to full density. After milling,the porous restoration design is sintered to full density to produce afinal restoration. Disadvantageously, the separate steps of milling theporous ceramic dental design and sintering the milled shape to form thefinal restoration, may preclude dentists from making chair-side ceramicrestorations, increasing the amount of time a patient must wait forrepair.

To reduce the amount of time to make a chair-side restoration,US2006/0204932 discloses an assemblage or library of “smart” mill blankspre-configured into geometries and sizes that closely resemble the finaldental parts. Material waste may be reduced compared to traditional millblanks that have a single size and shape, which is desirable when usingprecious or semi-precious materials. The smart mill blank library isdescribed as comprising a series of blanks with geometries that differother than by scale, and preferably having at most, one symmetric plane.The blank is mounted in a shaping apparatus by the integrated holderthat has an orientation-specific attachment key for the milling machine.

Methods of making ceramic restorations from near net shape millableblanks are also known, for example, from commonly owned U.S. PatentApplication Pub. No. 2013/0316305, which is hereby incorporated byreference in its entirety. A kit is disclosed containing millable blanksof various shapes, each shape designed to closely replicate arestoration shape thus minimizing material removal in chair-sideprocesses. The kit comprises a variety of shapes and shades ofrestoration blanks, as well as chair-side software, and a chair-sidemilling machine to convert millable blanks into finished, contouredrestorations by a dentist.

SUMMARY OF THE INVENTION

A machinable preform is described that is shapeable into a dentalrestoration with sufficient strength for anterior or posteriorapplications without the need for a further processing step tostrengthen the dental restoration after it has been shaped. A sinteredpreform may be shaped into a colored final, custom dental restoration.The sintered preform comprises a body of sintered material and a stemprojecting from the center of the preform body. Using a chair-sidemachine, a dental restoration may be shaped from the sintered preform,requiring no sintering after shaping, significantly reducing the time tocreate a custom finished product. The sintered preform has a unique sizeand shape that accommodates most custom restoration designs, and reducesthe amount of sintered material to be removed during shaping a dentalrestoration. Advantageously, the placement of the stem facilitates aunique tool path and machining strategy for shaping dental restorationsfrom sintered materials with no further sintering after shaping a dentalrestoration.

A method for making a sintered preform is also disclosed that comprisesthe steps of obtaining unsintered material, shaping the unsinteredmaterial into an unsintered shaped form having a body and a stem, andsintering the shaped form to full density to form the sintered preform.Unsintered material obtained in the form of a single block may be milledinto an unsintered, unitary continuous shaped form. Alternatively,unsintered material is molded by injection molding, into the unsinteredshaped form. In another alternative, unsintered material is molded intoan intermediate shaped form, and then milled for further shaperefinement prior to sintering to a final shaped form. In one embodiment,a pre-sintered ceramic block is milled into an intermediate shaped form,and then sintered to full density to form the sintered preform which maybe shaped by a chair-side mill into a finished dental restoration.Advantageously, the preform stem facilitates a unique tool path andmachining strategy for shaping a dental restoration from the sinteredbody. A unique tool is also described for use in shaping the sinteredpreform into dental restorations.

Custom dental restorations may be designed using known CAD(computer-aided design) processes, and CAM applications are applied tocreate a milling path and machining strategy based on the positionalinformation of the nested restoration design to machine the disclosedpreform into the final dental restoration. Unique nesting strategies maybe developed based on the preform shape and stem position that result inefficiencies in tool paths and machining strategies when machining acustom restoration from a sintered preform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A. A bottom perspective view of a preform according to oneembodiment.

FIG. 1B. A restoration design nested within one embodiment of a preformviewed from the bottom perspective view.

FIG. 1C. An illustration of dimensions of a block.

FIG. 2A. A top perspective view of a preform according to oneembodiment.

FIG. 2B. A side view of a preform according to one embodiment.

FIG. 2C. A front view of a preform according to one embodiment.

FIG. 3A. A bottom view of a preform according to one embodiment shownattached to a mandrel.

FIG. 3B. A top view of a preform according to one embodiment shownattached to a mandrel.

FIG. 3C. A side view of a preform according to one embodiment shownattached to a mandrel.

FIG. 4. A prospective view of a restoration according to one embodimentthat is made from a preform, and the preform stem.

FIGS. 5A and 5B. A bottom perspective view and side view, respectively,of a preform according to one embodiment.

FIGS. 6A and 6B. A front perspective view and a side view, respectively,of a grinding tool according to one embodiment.

DETAILED DESCRIPTION

A machinable preform is disclosed that may be shaped chair-side into afinal dental restoration, such as a crown, that has sufficient materialhardness and strength for insertion directly into the mouth of a patientwithout requiring sintering after shaping. With reference to anembodiment illustrated in FIGS. 1A and 1B, a machinable preform (100)comprises a preform body (101) and a preform stem (102) that projectsfrom the preform body (101). As exemplified in FIG. 1B, a dentalrestoration design (103) has full rotation within a model of the preformbody for selected nesting and placement of the dental restoration design(103) relative to the preform stem (102). The position of the stem onthe final dental restoration upon shaping the restoration from themachinable preform is determined by the nesting position.

In one embodiment illustrated in FIGS. 2A, 2B, and 2C, a preform (200)has a circular-cylindrical body (201) having a cylinder body length(line A-A′). The preform body (201) comprises a curved outer surface(204) and a center portion (205) between a top end (206) and a bottomend (207). In FIGS. 2A, 2B, and 2C, the cylindrical body (201) length(line A-A′) is oriented substantially orthogonally to the stem (202)length (FIG. 3A, line C-C′). The stem (202) projects away from thecurved outer surface (204) of the cylindrical body, and extends to anattaching member (203) for direct or indirect attachment to a shapingmachine. The preform body (201) further comprises a cavity (208)extending from a cavity breakout dimension in the bottom end (207)toward the center portion (205). The curved outer surface (204) of thecylindrical body center portion (205) as exemplified in FIGS. 2A, 2B and2C is substantially smooth, with the center portion (205) having auniform outer diameter between top and bottom ends.

FIGS. 3A, 3B and 3C illustrate an embodiment of a preform (300). Thecylindrical preform body (301) comprises circular top end face (309) andbottom end face (310), a center portion (306) with a substantiallycircular cross-section having outer diameter (line B-B′), and aconcavity (311) from which a cavity (308) extends inwardly towards thecenter portion (306). The stem (302) extends away from the curvedpreform body outer surface (304) of the center portion approximatelyequidistance between top and bottom ends. The stem extends between thecurved center portion (306) and an attaching member (303). The attachingmember (303) attaches to a mandrel (305) by the attaching member bottomsurface (315), and provides indirect attachment of the preform to ashaping machine.

The preform body from which the restoration crown is shaped may comprisea center portion in the form of a cylinder, as depicted in the Figures,though other shapes may be suitable for use herein. Alternately, thebody (101) or body center portion, comprises, for example, an ellipsoidcylinder, a polyhedron, curved polyhedron, a cylinder with flattenedsurfaces, a cube, a cube with rounded edges, and the like. FIG. 1Billustrates an embodiment in which the shape and size of a preform body(101) accommodates complete rotation of a restoration design (103)within the preform body around the z-axis (line Z-Z′); thus, the preformcomprises 360 degree (full rotation) stem (102) placement relative tothe final restoration. The outer diameter of the circular cross-sectionof the center portion from which the restoration design is shaped may befrom about 12 mm to about 20 mm, or from about 13 mm to about 18 mm, orfrom about 14 mm to about 17 mm. The length of the preform body betweenthe top end and the bottom end is sufficient to accommodate the heightof most dental restoration designs (400) when measured, for example,from the highest point of the occlusal surface (404) to the lowest pointon a tooth margin (405); thus, the length of the preform body or thecenter portion of the preform body may be less than 20 mm, or less than18 mm, or less than 16 mm, or less than 15 mm, or may be between about10 mm and 15 mm. In some embodiments, the ratio of the cross-sectionaldiameter of the center portion to the length of the preform body isgreater than 1.0:1.0.

In some embodiments, a preform body having a non-circular cross-sectionor an irregular shaped cross-section, has a cross-sectional geometrywithin the center portion for full rotation (360 degrees) of therestoration design around the z-axis. A preform body comprising a topportion, a bottom portion, and a center portion there between has across-sectional geometry (approximately parallel with top and bottomsurfaces) with an inscribed circle diameter greater than approximately12 mm and a circumscribed circle diameter less than approximately 20 mmat a location where the stem projects from the center portion. Incontrast, a representation of a block having a size and shape of knownmilling blocks (e.g. approximately 15 mm×16 mm) has a cross-sectionalgeometry (112) as illustrated in FIG. 1C. In this example, an inscribedcircle (114) of a selected diameter (e.g. 12 mm) fits within thecross-sectional dimensions of the representative block. However, thecross-sectional geometry of the representative block does not fit withina circumscribed circle (115) of a selected diameter (e.g., about 20 mm)thus, reducing the size of the crown design that may be fully rotatedwithin the known block design without increasing the size of the block.

In some embodiments, the preform body has flat end faces and a uniformcross-sectional diameter or width throughout the length of the body.Alternatively, top and bottom end regions (206, 207) taper to comprisetop and bottom end faces with smaller cross-sectional diameters orwidths than the center portion (205). Tapered top and/or bottom endportions may comprise a shaped edge between the preform outer surface(204) of the center portion and an end face (e.g., bottom end face 211),or a shaped edge around a cavity (208) on the end face, or both. Forexample, as illustrated in FIG. 1A, the bottom end region (105) has afirst filleted edge (106) between the outer surface (104) of the centerportion and a bottom end face (107); further, a second filleted edge(108) surrounds a concavity (109) on the bottom end face (107), and acavity (110) extends inwardly from the second filleted edge toward thecenter portion (111) of the cylindrical body. In the embodimentillustrated FIG. 1A, preform body tapers to a top end region (113) thatcomprises a filleted edge between the top end face (not illustrated) andthe preform body outer surface (104).

A further preform (500) is exemplified in FIG. 5A (bottom view) and FIG.5B (side view) which illustrate a preform body (501) comprising a stem(502) and an attachment member (503) wherein both a bottom end region(506) and top end region (507) have chamfered edges (508, 508′). Thepreform body outer surface (504) diameter tapers from the center portion(505) to the bottom and top end faces (509, 510, respectively). Thebottom end region (506) exemplified in FIG. 5A has second chamfered edge(511) forming a concavity (512) on the bottom end face (509), and acavity (513) extends from the concavity toward the center portion.

In some embodiments, a machinable preform having one or more shapededges has less material to be removed when making a final dentalrestoration, such as a crown. Shaped edges around the cavity mayfacilitate access to the cavity by a shaping tool. Further, a cavitysubstantially devoid of preform material may reduce the amount ofmaterial to be removed when shaping the restoration. As exemplified inFIG. 5A, the cavity (513) extends from the cavity opening into thecenter portion of the preform body from a concavity on the top and/orbottom end faces forming an inner surface. In other embodiments, acavity is formed in each of the front end and back end of a preform.

The shape of each cavity may be the same or different, and may comprise,but is not limited to, an inverted cone, dome, cylinder, trough, or thelike, or may have an irregular shape. An opening or breakout geometry ofthe cavity may have a width (or diameter, for example where the breakoutarea is circular) that is between about 20% and 80% of the outerdiameter or width of the center portion of the preform body. For ease ofdiscussion, the term width as used herein may refer to diameter, aswell. In some embodiments, the opening of the cavity has a width that isabout 30% to about 75%, or between about 40% to about 75% of the outerwidth of the center portion of the preform body, or between 50% and 80%of the outer width of the center portion, or the cavity opening orbreak-out dimension has a surface area that is about 50% to about 80% ofthe surface area of a top end face, a bottom end face, or a centerportion cross-section.

The approximate cavity depth may be between 5% and 50% of the length ofthe preform body, or between 10% and 35% of the length of the preformbody, or between 10% and 30% of the length of the body, when the preformis measured from top end to bottom end. A circular cavity opening mayhave an inner diameter of up to about 75 percent of the outer surfacediameter of the preform body when measured from the end face.

In some embodiments, the preform body has an inner surface (212) in theapproximate shape of an inverted cone, formed by the cavity. The innersurface is accessed by the machining tool and machined to form theconcave surface of the dental restoration that attaches to and abuts thestructure in the patient's mouth. By nesting a restoration design withina model of the preform body, and coaxially aligning the cavity of thepreform with the restoration design concave inner surface, the amount ofmaterial removal during the shaping process is reduced.

In one embodiment, the preform body comprises a cross-section width(which as discussed above may refer to a diameter) and length thataccommodates the size of at least about 90% of all single anterior andposterior (for example, 1^(st) and 2^(nd) molars and bicuspids) dentalrestorations, eliminating the need for dentists to accumulate aninventory of multiple sizes and shapes of preforms. The preform body maybe designed based on information regarding previously preparedrestoration designs of different shapes and sizes. In one embodiment, apreform body is designed by electronic representations of thousands ofsingle crown restoration designs that are obtained, and then overlaid sothat convex inner surfaces of the restoration designs are orientedaround a common axis (for example, as shown in FIG. 1B). In oneembodiment, the preform body design is a composite of restorationdesigns of multiple anterior teeth types (e.g., central incisors,lateral incisors, cuspids, and optionally, 1^(st) and 2^(nd) bicuspids).In another embodiment, the preform body design is a composite ofrestoration designs of multiple posterior teeth types (e.g., 1^(st) and2^(nd) molars, and optionally 1^(st) and 2^(nd)bicuspids). In anotherembodiment, the preform body design is a composite of restorationdesigns of anterior and posterior teeth types.

The overlaid restoration designs, coaxially aligned, are rotated arounda common axis. Upon rotation, the largest dimensions of the compositedesigns, for example, the restoration design parting lines orsilhouette, form the greatest outer surface dimension of a shaped bodydesign. In some embodiments, the outer surface of the shaped body designmay be smoothed based on the greatest outer surface dimensions to form asubstantially cylindrical shape and circular cross-section having adiameter of suitable size to nest about 90% of the restoration designswith 360 degree rotation around a center axis (e.g., line Z-Z′). Preformedges between bottom and top ends, and the center portion may be shapedas described above. Optionally, a preform cavity design corresponds tothe composite restoration design concave surface that is smoothed toprovide an inverted cone-shaped inner surface in the preform body.

In one embodiment, a preform body having a rounded or circularcross-sectional design is provided that has less material volumecompared to a standard preform block shape having a cube or rectangularprism shape with approximately 90 degree angles at the edges or cornersthat accommodates a similarly sized composite restoration design.Moreover, a single preform design that accommodates full rotation of acomposite restoration design around the z-axis, is in contrast to nearnet shape blocks having asymmetric geometries that mimic or resembleasymmetric tooth features. The near net shaped blocks with toothfeatures do not accommodate rotation of restoration designs, and requirea large library or kit of specific tooth types or tooth numbers toensure fitment of a range of possible restoration types and sizes.

The stem provides support for the preform body during shaping of thefinal restoration. The stem length may provide a sufficiently largespace between the preform body and an attaching member to allow forplacement of a grinding tool in a position adjacent the preform body forentry into a tool path without contacting preform material. In anembodiment exemplified in FIGS. 3A and 3B, the stem (302) bridges thecylindrical body (301) and attaching member (303), and extends generallyorthogonally from the cylindrical body outer surface approximatelymid-way between the top end and bottom end. The stem may project fromthe outer surface of a preform equidistance from a top end face and abottom end face, or within about 10% to about 25% of halfway between atop end face and a bottom end face. In other embodiments, the distanceof the stem connection to the top end or bottom end is equal to about20% to about 80% of the preform body length, or about 25% to about 75%of the length of the preform body, or a distance that is equal to about30% to 70% of the length of the body, or a distance that is equal toabout 40% to 60% of the length of the body.

The axis of the stem length (line C-C′) may be substantially orthogonalto the axis of cylindrical body (301) length (line A-A′). In someembodiments, the stem length axis is within about 30 degrees or withinabout 45 degrees from orthogonal, relative to the preform body length.The shape of the stem may be a cylinder, cone, prism or the like. In oneembodiment, a preform body that tapers to shaped edges at the front andback ends, comprises a stem that extends from the center portion of thepreform body, and after machining, the stem is connected to the middleof a final restoration (400), away from the occlusal surface and theedge or margin of the final restoration, as seen in FIG. 4.

In one embodiment, the preform body is a fully sintered material and theflex strength of the stem (302) at the first end (313) is sufficientlyhigh to support the sintered preform (300) during machining from asintered state, and sufficiently low for the finished restoration toeasily be broken off from the stem, for example, by hand. The stem (302)of a preform (300) remains attached to and supports the sinteredcylindrical body (301) at the first end (313) throughout the shapingprocess until a finished restoration is obtained. In contrast to thepreform bodies described herein having a stem (302) extending from theouter surface of the shapeable preform body, traditional restorationmilling processes produce sprues or connectors during the shapingprocess that are remnants of unsintered block materials.

In some embodiments, prior to shaping the restoration, the length of thepreform stem is greater than the stem width at the first stem end (313)proximate the preform body. The stem length may be between about 3 mmand about 12 mm, or between about 3 mm and 10 mm. In some embodiments,the stem length may be greater than about 3 mm, or greater than about 4mm, or greater than about 5 mm, or greater than about 6 mm, or greaterthan about 8 mm. In one embodiment, the width (for purposes herein,‘width’ may also be used to refer to a stem diameter) of the first stemend proximate the cylindrical body is less than the width (diameter) ofthe second stem end (314) proximate the attaching member (303). Thefirst stem end width may be in the range of 1 mm to 5 mm, or about 1 mmto about 4 mm, or about 1.5 mm to about 3.5 mm, or 1.5 mm to about 3 mm,or less than or equal to about 4 mm, or less than or equal to about 3mm, or less than or equal to about 2.5 mm, or less than or equal toabout 2 mm.

In some embodiments, the ratio of stem length to the first stem endwidth (the end proximate the cylinder body) is greater than or equal to1.5:1, or greater than 2:1, or greater than 3:1, or greater than 3.5:1,and less than 6:1, or less than 5:1, or less than 4.5:1, or less than orequal to about 4:1. In one embodiment, the stem has sufficient length toprovide access and placement of a machining tool between the attachingmember and the cylindrical body, without the tool contacting the preformmaterial, to reduce wear on the machining tool when machining thecylindrical body near the stem. Thus, in this embodiment, the stemlength is greater than the diameter of the tool tip, tool shank or both.

The attaching member (303) is joined to the stem at the second end ofthe stem (314) and secures the machinable preform directly to a shapingmachine, or indirectly to an intermediary component (such as a mandrel305) during the shaping process. The shape and size of the attachingmember may be compatible with any machine or intermediary mandrelsuitable for shaping the sintered preform to a final dental restoration.The attaching member may secure the sintered preform directly orindirectly to the machine by a mechanical means including, a clamp, agrip, adhesive or other mechanical attachment. For example, theattaching member having a substantially flat bottom surface (315) shapedas a square, rectangular, or circle may be adhesively attached to amandrel, as exemplified in FIG. 3A. In another embodiment (not shown),the sintered preform comprises an attaching member that is insertablymountable into a mandrel, and is secured by gripping or clamping theattaching member in the mandrel. In one example of this embodiment, thestem is lengthened and the second stem end is shaped to insert into amandrel. The attaching member may comprise a mechanical attaching meansfor attachment to a mandrel or directly to a shaping machine, such as ahole (316) for placement of a screw, or a dove tail.

Preform materials may include those having a Vickers hardness valuegreater than or equal to about 4HV GPa (Macro Vickers Hardness), or avalue in the range of 4HV GPa to 20HV GPa, when measured according tothe method provided herein. Alternatively, preform materials have aVickers Hardness value between 5HV GPa and 15HV GPa, or between 11HV GPaand 14HV GPa. Preform body materials comprising hardness values withinthis range may include metals, such as cobalt chrome, glass and glassceramics, such as lithium silicate and lithium disilicate, and ceramics,including sintered ceramics comprising alumina and zirconia. Dentalrestoration materials, including but not limited to commerciallyavailable dental glass, glass ceramic or ceramic, or combinationsthereof, may be used for making the machinable preforms describedherein. Ceramic materials may comprise zirconia, alumina, yttria,hafnium oxide, tantalum oxide, titanium oxide, niobium oxide andmixtures thereof. Zirconia ceramic materials include materials comprisedpredominantly of zirconia, including those materials in which zirconiais present in an amount of about 85% to about 100% weight percent of theceramic material. Zirconia ceramics may comprise zirconia, stabilizedzirconia, such as tetragonal, stabilized zirconia, and mixtures thereof.Yttria-stabilized zirconia may comprise about 3 mol % to about 6 mol %yttria-stabilized zirconia, or about 2 mol % to about 7 mol %yttria-stabilized zirconia. Examples of stabilized zirconia suitable foruse herein include, but are not limited to, yttria-stabilized zirconiacommercially available from (for example, through Tosoh USA, as TZ-3Ygrades). Methods form making dental ceramics also suitable for useherein may be found in commonly owned U.S. Pat. No. 8,298,329, which ishereby incorporated herein in its entirety.

Unsintered materials may be shaped into an intermediate form havingsubstantially the same geometry as the sintered preform, but withenlarged dimensions to accommodate shrinkage upon sintering, wherenecessary. The intermediate shaped form may be made by injectionmolding, or milling, or grinding unsintered materials. Suitableunsintered ceramic materials include ceramic powders and ceramic blocksthat have not been fully sintered to theoretical maximum density.Ceramic powders may be made into blocks by processes including moldingand pressing, including biaxial or iso-static pressing, and mayoptionally comprise binders and processing aids. Optionally, ceramicpowder may be processed into blocks by slip casting processes, includingprocesses described in commonly owned U.S. Patent Publication Nos.2009/0115084; 2013/0231239; and 2013/0313738, incorporated by referencein their entirety.

Colored materials may be used to make shaded machinable preforms havingthe color of natural or artificial dentition, requiring no furthercoloring after formation of the dental restoration. Coloring agents maybe incorporated during powder or block formation to more closely matchthe appearance of natural or commercially available artificial dentitionthan uncolored or unshaded ceramic materials. For example, U.S. PatentPublication 2013/0231239, describes methods for coloring ceramics bycolloidal dispersion and casting the ceramics by slip casting methods,and is incorporated by reference herein, in its entirety. A furtherexample includes US Patent Publication 2014/0109797, which teachesmethods for making colored ceramic powder, formed into green stateceramic bodies by isostatic or biaxial press manufacturing processes isalso incorporated by reference herein in its entirety. Optionally,coloring agents may be mixed directly with ceramic powders for example,as metallic salts, coloring liquids, or colorized powders, prior topressing into blocks. Optionally, intermediate preform shapes made fromporous materials are shaded, for example, by dipping into coloringliquids, and then sintered.

Unsintered materials include green state and pre-sintered ceramic blocksmade by the above processes that may be heated or partially sinteredreducing porosity to facilitate shaping without chipping or breakage.Pre-sintered blocks are hard enough to retain structure for milling intoshaped forms, yet soft enough to allow for rapid shaping without damagethe milling tool, and have not been heated or sintered to full density.Pre-sintered blocks useful in the methods described herein includeporous blocks that may have a density in the range of about 50% to about90%, or 50% to 95%, of the theoretical maximum density of the fullysintered ceramic material. It should be noted that the pre-sintereddensity may include non-ceramic binder as well as ceramic particles,compared to the theoretical pore-free density of a fully sinteredceramic block. In some embodiments, the theoretical maximum density offully sintered zirconia ceramics is between about 5.9 g/cm³ to about 6.1g/cm³, or for example, or about 6.08 g/cm³. Pre-sintered blocks suitablefor use in making intermediate shaped forms include commerciallyavailable ceramic milling blocks including those sold under the tradename BruxZir® (for example, BruxZir® Shaded 16 Milling Blanks, GlidewellDirect, Irvine, Calif.).

Upon sintering, porosity in pre-sintered ceramic blocks results inshrinkage that may be calculated from the material density with highlypredictable shrinkage. Thus, an intermediate shaped form may be largerthan the final preform by a scaled factor that anticipates the reductionin size upon sintering to full density. Likewise, intermediate shapedforms made by injection molding unsintered ceramic materials that shrinkupon sintering, are designed with an enlargement factor that anticipatessize reduction upon sintering. CAD/CAM processes may be used to designthe intermediate shaped form, and send corresponding millinginstructions for milling with a scaled enlargement factor. Intermediateshaped forms can be milled with commercially available mills and millingtools, for example, as specified by the manufacturer according to therequirements of the ceramic milling blocks.

A unitary or monolithic preform comprising the preform body, stem and,optionally, attaching member, are shaped from a single continuousgreen-state block or pre-sintered ceramic block, requiring no separateattachment step for attaching the stem and/or attaching member to thepreform body. Alternatively, the stem and attaching member may be madeas a unitary structure and attached to the preform body as a separatestep. In another embodiment, the shaped preform is made by known moldingprocesses, including injection molding, to form a unitary or monolithicpreform comprising the preform body, stem and, optionally, the attachingmember as a continuous structure. Alternatively, a shaped form may bemade by a combination of molding and milling techniques, for example,where an intermediate shaped form is first molded, and then the stemand/or attaching member is milled by standard milling techniques.Alternatively, the stem and attaching member may be separately attachedto a preform body, before or after sintering.

The intermediate shaped form may be sintered to a density greater thanabout 95% of the theoretical maximum density by known sinteringprotocols. For sintering ceramic preforms, such as zirconia ceramicpreforms, to densities greater than about 95%, or greater than about 98%or greater than about 99%, or greater than about 99.5%, of the maximumtheoretical density of the ceramic body, material manufacturingprotocols suitable for sintering dental restorations may be used. Forexample, an intermediate shaped form milled from a pre-sintered zirconiablock may be sintered at a temperature between about 400° C. and 1700°C., for between about 30 minutes and 48 hours, or according to thesintering protocol provided by the manufacturer of ceramic blocks toform a sintered zirconia preform having a density in the range of about5.8 g/cm³ and 6.1 g/cm³, such as 6.08 g/cm³, or in a range of about 5.9g/cm³ and 6.0 g/cm³.

The preform body comprises materials that may be shaped as dentalrestorations and that have sufficient strength properties to beacceptable for use in anterior, posterior or both anterior and posteriordental restoration applications, without additional post-shapingprocessing steps to alter the material strength properties aftershaping, such as by sintering. Sintered preforms may comprise zirconiaceramic materials that have high flexural strength, including strengthvalues greater than about 400 MPa, or greater than about 500 MPa, orgreater than about 600 MPa, or greater than about 800 MPA, when testedby a flexural strength test method for zirconia materials as outlined inISO 6872:2008, as measured and calculated according to the 3 pointflexural strength test described for Dentistry-Ceramic Materials.

One method of making a machinable preform for use in dental restorationsis disclosed that comprises the steps of a) obtaining an unsinteredzirconia ceramic material; b) shaping an unsintered shaped form from theunsintered zirconia ceramic material, wherein the unsintered shaped formcomprises: a cylindrical body of unsintered ceramic material comprisinga top end, a bottom end, and a center portion between the top and bottomends; and a cavity in at least one of the top or bottom ends; a stemthat projects from the outer surface of the center portion of thecylindrical body; and c) sintering the unsintered zirconia ceramicshaped form to achieve a density of about 98% to about 100% percent ofthe theoretical maximum density of the zirconia ceramic body to form themachinable sintered preform.

In one embodiment of the method, the unsintered zirconia ceramicmaterial is a single pre-sintered ceramic block, and the step of shapingthe unsintered ceramic shaped form comprises milling the zirconiapre-sintered ceramic block into a monolithic shaped form comprising abody portion and a stem as a continuous structure. In anotherembodiment, the step of shaping the unsintered ceramic form comprisesmolding the unsintered ceramic material into the monolithic shaped form.In one embodiment, the pre-sintered zirconia ceramic shaped formcomprises a cylindrical body and a stem having a first size, and issintered to form a sintered zirconia preform comprising a cylindricalbody and a stem having a reduced second size.

Sintered zirconia preforms that have been fully sintered are shaped intoa finished dental restoration based on a CAD design using a CNC machineand a grinding tool. A final dental restoration (400) made from thesintered preform is exemplified in the illustration of FIG. 4. Arestoration crown (401) shaped from the sintered preform is shown priorto removing the stem (402), which extends from a crown outer surfacebetween the margin of the tooth and the occlusal (biting) surface. Insome embodiments a minimal amount of sintered preform material (403)remains from the cylindrical body between the final dental restorationcrown (401) and the stem (402) which may be removed upon removal of thestem, for example, by hand-sanding. In one embodiment, a single grindingtool may be used to shape a fully sintered zirconia preform into afinished restoration in less than about 60 minutes.

Dental restorations may be shaped by grinding tools instead oftraditional milling tools that are unsuitable for shaping detaileddental restorations from materials having high hardness values. Agrinding tool (600), for example as illustrated in FIG. 6, comprises ashank (601) that comprises an embedded diamond coating (603) on theshank and tip (602). Diamonds suitable for use herein include blocky orfriable diamonds having an average size in the range of about 90 micronto about 250 micron, or an average size in the range of about 107 micronto about 250 micron, or an average size in the range of about 120 micronto about 250 micron, or for example, an average size in the range ofabout 120 micron to about 180 micron. Suitable diamond coatings includethose with diamonds embedded by a metal alloy layer for more than halfthe average height of the diamond, for example, as determined by SEManalysis. In some embodiments, grinding tools have a diamond coatedshank having a metal alloy layer having thickness that is greater thanabout 50%, or greater than about 60%, or greater than about 70%, orgreater than about 80%, or between about 60% and 90%, of the averagediamond height. Grinding tools having a coating of diamonds that areembedded to a depth of about 50% to 95% of the average diamond size orheight, or about 60% to about 95% of the average height of the diamond,or about 80% to about 90% of the height of the diamond height are usefulfor shaping machinable preforms, such as fully sintered zirconiapreforms, having the hardness values described herein. In oneembodiment, a grinding tool has a diamond coated shank comprising anaverage diamond grit size in the range of 126 grit to 181 grit, and anickel alloy layer having a thickness that is greater than or equal toabout 70% of the average diamond height.

A kit is provided for forming a dental restoration that comprises agrinding tool and a machinable preform, wherein the preform material hassuitable strength and hardness values for use as a posterior dentalrestoration crown without the need for post-shaping treatment to modifystrength properties of the dental restoration that is shaped from it.The machinable preform comprises a preform body and a stem that extendsgenerally orthogonally from the preform body length. The single grindingtool may be used to shape the preform body into a final dentalrestoration, and the grinding tool comprises a diamond-coated shankcomprising diamonds with an average size in the range of 107 micron to250 micron, embedded in a metal alloy layer having a thickness that isin the range of about 60% to 95% of the diamond height. In oneembodiment, the preform body comprises a pre-shaded material that hasbeen selected, for example, to match existing dentition or a shade guidecolor, and requires no post-shaping coloring or sintering.

In a further embodiment, a plurality of similarly shaped preform bodiesare provided in a multiplicity of colors suitable for use in makingdental restorations in a range of dental shades that require nopost-shaping coloring or sintering, such as shades corresponding to theVITA classical shade guide, the ADA guidelines, or other commerciallyrecognized shade guide colors suitable for use in the dental industry.

Test Methods

Flexural Strength Test

Flexure tests were performed on sintered test materials using theInstron-Flexural Strength test method for zirconia materials as outlinedin ISO 6872:2008.

Test bars were prepared by cutting pre-sintered materials taking intoconsideration the targeted dimensions of the sintered test bars and theenlargement factor (E.F.) of the material, as follows:

starting thickness=3 mm×E.F.;

starting width=4 mm×E.F.;

starting length=55 mm×E.F.

The cut, pre-sintered bars were sintered substantially according to thesintering profile listed above for the Translucency Test. Flexuralstrength data was measured and calculated according to the 3 pointflexural strength test described in ISO (International Standard) 6872Dentistry-Ceramic Materials.

Vickers Hardness Number

Preform materials may be tested for hardness using a Vickers hardness(macro-hardness test). Hardness numbers (HV) may be calculated asdescribed in ISO-6507, or by determining the ratio of F/A where F is theforce applied in kg/m² and A is the surface area of the resultingindentation (mm²). HV numbers may be converted to SI units and reportedin units, HV GPa, as follows: H(GPa)=0.009817HV.

Examples 1-21

Twenty-one zirconia crowns of multiple tooth types (numbers) were shapedfrom sintered zirconia preforms by the methods described herein.

Pre-sintered zirconia milling blocks were obtained (BruxZir® Shadedmilling blocks, Glidewell Direct, Irvine, Calif.) and milled intocylindrically shaped preforms. The preforms having a circularcross-section were milled by standard milling procedures incorporatingan enlargement factor calculated from the block density. Theintermediate pre-sintered shaped forms had a cylindrical body, with atop and bottom end, a stem having a first stem end equidistance from thetop and bottom ends, the stem extended orthogonally from the centerportion relative to the length of the cylindrical body, an attachingmember attached to a second stem end, substantially as depicted in FIG.1A. The preforms had a cavity extending inwardly from a bottom end face.The stem had sufficient length between the attaching member and thecylindrical body for positioning the tip of a ball nose grinding tool inthe z-axis direction without contacting the sintered preform. Theattaching member shape and size was compatible for attaching to amandrel used with the CNC machine in the grinding process.

The pre-sintered shaped forms were sintered according to sinteringprofile of the zirconia blocks provided by the manufacturer to formfully sintered zirconia preforms having a density between about 5.9g/cm³ and 6.1 g/cm³. The fully sintered preforms had a body lengthbetween about 12.8 mm and 14.2 mm, a cross-sectional outer diameter ofabout 14 mm to 15 mm, a cavity breakout diameter on the bottom endsurface having a diameter of about 7 mm to 8 mm, a first stem end widthof about 2-2.8 and a stem length between about 6.8 and 7.3 mm.

The fully sintered zirconia preforms made according to theabove-procedure were glued to metallic mandrels and grinded intomultiple tooth shapes based on CAD design files using a 3+1 axes CNCmachine (z, x, and y axes direction movement of grinding tool, plusrotation of the preform between tool path cycles). The grinding toolcomprised diamonds (size: 126) embedded in nickel plating to about80%-90% of the average diamond height. A CAM lacing cycle, with stepover capability in both planar and the grinding tool axial direction,was utilized to grind all surfaces of the crown. The grinding tool had agrit size between about 90-210.

An air spindle with rotational speed of about 150000 rpm and a minimuminlet air pressure of about 85 psi were used to grind the fully sinteredzirconia preform. The CAM lacing cycle parameters were determined basedon tooth number, and grinding surface of the sintered preform relativeto the top or bottom of the restoration, the stem side or the sideopposite the stem side of the restoration). Restorations were made fromthe sintered preforms for posterior teeth numbers 2, 3, 14, 15, 18, 19,30 and 31 in under 60 minutes as seen in Table 1.

TABLE 1 Time to completed restoration from Sintered Preform in Minutes.Time (in Minutes) for each Example Number Tooth # example 1-4 2 52; 47;51; 44 5-8 3 53; 49; 54; 57  9-10 14 47;51 11 15 38 12-14 18 54; 55; 4715 19 49 16-19 30 53; 48; 50; 50 20-21 31 49;50

FIG. 4 illustrates a restoration made according to the Examples havingminimal residual material remaining between the stem and restorationafter grinding is completed. The restoration crown may be snapped off ofthe stem.

Examples 22-25

Zirconia crowns were shaped from sintered zirconia preforms that weremade substantially according to the methods provided in Examples 1-21.

A restoration design was created for a second molar (tooth number 15)using a dental CAD system (IOS™ FASTDESIGN) based on patient scan data.The restoration design was nested within a computer model of a preform.The size and shape of the preform provided full rotation of therestoration design around the z-z′ axis providing alternate locations onthe crown restoration design for positioning the preform stem. Therestoration design was nested four times to provide four nested designs,and in each nesting operation, the stem of the preform was positioned inone of four different locations, as indicated in Table 2. Nestingpositions included placement of the Y-axis of the stem between mesialcontact area and buccal surfaces (mesio-buccal), between distal contactarea and buccal surfaces (disto-buccal), between distal contact area andlingual surface (disto-lingual), and between mesial contact area andlingual surface (mesio-lingual).

The sintered zirconia preforms were grinded using grinding tools andmethods substantially according to Examples 1-21. As reported in Table2, dental restorations were successfully produced from the sinteredzirconia preforms in two out of the four nested designs. The successfulrestorations were produced in under one hour.

TABLE 2 Forming A Dental Restoration From Multiple Nesting Options.Successful Time (minutes) to Ex.# Stem Location Completion Completion 22Mesio-buccal Yes 38 23 Disto-buccal No — 24 Disto-lingual Yes 42 25Mesio-lingual No —

The shape and size of the preform design provided full rotation of thedental restoration design within the geometry of the preform bodyproviding multiple nesting opportunities to enable successful shaping ofa dental restoration from a fully sintered ceramic preform whilereducing the amount of sintered material to be removed during thegrinding process.

We claim:
 1. A machinable preform for shaping a dental restorationcomprising: a body comprised of a machinable dental material having aVickers hardness value in the range of 4HV GPa to 20HV GPa, comprisingan outer surface, a top end, a bottom end, and a center portion betweenthe top end and the bottom end; a stem that projects from the outersurface of the center portion of the body from a first stem end having awidth less than or equal to 4 mm; and optionally, an attaching memberconnected to the stem at a second stem end, for attaching the sinteredceramic preform to a shaping machine during shaping, wherein the centerportion of the body has a cross-sectional geometry having an inscribedcircle diameter greater than 12 mm and a circumscribed circle less than20 mm at the location of the first stem end.
 2. The machinable preformof claim 1, wherein the center portion comprises a cylindrical body anda circular cross-sectional geometry that has a diameter less than 20 mmat the location of the first stem end.
 3. The machinable preform ofclaim 1, wherein a ratio of the stem length to the first stem end widthis greater than 1.5:1 and less than or equal to 4:1.
 4. The machinablepreform of claim, 1 further comprising a cavity contained within theouter surface of the preform body that extends from a bottom end surfacetowards the center portion.
 5. The machinable preform of claim 1,wherein the machinable dental material comprises ceramic, glass or glassceramic.
 6. The machinable preform of claim 1 wherein the machinabledental material comprises a sintered zirconia ceramic material that is85% by weight or more of fully sintered zirconia or fully sinteredyttria-stabilized zirconia.
 7. A machinable preform for shaping a dentalrestoration comprising: a body of fully sintered ceramic dental materialcomprising a top end and a bottom end, a cylindrically shaped centerportion between the top end and the bottom end having an outer surfaceand a circular cross-section that has a diameter greater than or equalto 12 mm and less than or equal to 20 mm, a concavity in the bottom endforming a cavity opening having a surface area that is between about 50%and 80% of a bottom end surface; a stem that projects from the outersurface of the center portion; and optionally, an attaching memberconnected to the stem at a second stem end, for attaching the fullysintered ceramic preform to a shaping machine during shaping.
 8. Themachinable preform of claim 7, comprising at least one filleted edgebetween the outer surface of the cylindrically shaped center portion andat least one of the top and bottom ends.
 9. The machinable preform ofclaim 7, comprising at least one chamfered edge around the cavity. 10.The machinable preform of claim 7, wherein the cavity comprises a depththat is between about 5% and 50% of the length of the cylindrical body.11. The machinable preform of claim 7, wherein the distance of the stemfirst end to the top end is between 30% to about 70% of the length ofthe body.
 12. A method of making a machinable preform for use in shapinga dental restoration comprises the steps of: a) obtaining anintermediate preform material; b) shaping the intermediate preformmaterial to form an intermediate shape comprising: a body comprising anouter surface, a top end, a bottom end, and a center portion between thetop and bottom ends, and optionally, a cavity extending toward thecenter portion from at least one of the top and bottom ends; and a stemprojecting from the outer surface at the center portion between the topand bottom ends at a first stem end; and optionally, an attaching memberattached to a second stem end; and c) sintering the intermediate shapeto a density of about 98% to about 100% percent of the theoreticalmaximum density of the preform material to form a sintered preform,wherein the sintered preform stem first end has a width of less than orequal to about 4 mm, and wherein the center portion of the sinteredpreform body has a cross-sectional geometry having an inscribed circlediameter greater than 12 mm and a circumscribed circle diameter lessthan 20 mm at the first stem end.
 13. The method of claim 12 wherein theintermediate preform material is a pre-sintered ceramic block, and thestep of shaping the intermediate preform material comprises milling thepre-sintered ceramic block into a monolithic intermediate shape.
 14. Themethod of claim 12, wherein the step of shaping comprises molding theintermediate preform material into the intermediate shape.
 15. Themethod of claim 12, wherein the intermediate material upon sintering hasa Vickers hardness value in the range of 10HV GPa-15HV GPa.
 16. A kitfor making a finished dental restoration comprising: a. a preformcomprising i. a body for shaping a dental restoration comprising amaterial having a Vickers hardness value in the range of 4HV GPa-20HVGPa, the body comprising an outer surface, a top end, a bottom end, anda center portion between the top end and the bottom end; ii. a stem thatprojects from the center portion from a first stem end; and iii.optionally, an attaching member connected to the stem at a second stemend, for attaching the preform to a shaping machine during shaping; wherein the center portion of the body has a cross-sectional geometryhaving an inscribed circle diameter greater than 12 mm and acircumscribed circle less than 20 mm at the location of the first stemend, and b. a grinding tool comprising a shank and a diamond coatingcomprising diamonds embedded in a metal alloy coating having a thicknessin the range of 50% to 95% of the height of the diamonds.
 17. The kit ofclaim 16, wherein the preform body material is fully sintered zirconiaceramic that comprises a shade corresponding with natural or artificialdentition.
 18. The kit of claim 16, wherein the preform body is acylindrical body, comprising a circular cross-section at the location ofthe first stem end, and has a cavity extending from a top end, a bottomend or both, towards the center portion.
 19. The kit of claim 16,wherein the first stem end has a width or diameter that is less than orequal to about 4 mm.
 20. The kit of claim 16, wherein the distance ofthe stem first end to the top end or bottom end is equal to about 30% toabout 70% of the preform body length.